1. Field of the Invention
The present invention relates to a synthetic resin bearing which is disposed between two members which rotate relative to each other and is capable of smoothly supporting the radial load and the thrust load, the synthetic resin bearing being particularly suitable for use in a strut-type suspension (MacPherson type) in a four-wheel drive vehicle.
2. Description of the Related Art
In general, a strut-type suspension is mainly used on a front wheel of a four-wheel drive vehicle, and is arranged such that a strut assembly, which incorporates a hydraulic shock absorber in an outer cylinder formed integrally with a main shaft, is combined with a coil spring. As the structures of this type of suspension, there are the following two structures: (1) a structure in which the axis of the coil spring is positively offset with respect to the axis of the strut, and the sliding of the piston rod of the shock absorber incorporated in the strut is effected smoothly, and (2) a structure in which the axis of the coil spring is aligned with the axis of the strut.
In either structure, a bearing is disposed between a chassis-side mounting member and an upper spring seat of the coil spring to allow smooth rotation of the strut assembly when the strut assembly rotates together with the coil spring in a steering operation.
As the bearing, a rolling bearing using balls or a sliding bearing formed of a synthetic resin is used.
However, the rolling bearing has a problem in that fatigue failure can possibly occur in the balls due to very small vibrations, vibrational load, and the like, thereby making it difficult to maintain smooth steering operation over extended periods of time.
On the other hand, the sliding bearing formed of a synthetic resin has a problem in that since the coefficient of friction of the sliding bearing is high, the steering operation is made heavy.
Furthermore, in either type of the bearing, a dust seal formed of an elastomeric resilient member is fitted between surfaces of relative rotation for the purpose of preventing the entry of foreign objects, such as dust, onto the sliding surfaces. Since the dust seal is disposed between and in close sliding contact with the surfaces of relative rotation, there is a problem in that the sliding frictional force is high during the steering operation, so that the steering force is made heavy, and that particularly in the case of the sliding bearing formed of a synthetic resin, the steering force is made even heavier.
To overcome these problems of the conventional art, Japanese Utility Model Application Laid-Open No. 2-140022 (hereafter referred to as the prior art) discloses a synthetic resin bearing which is capable of maintaining a stable steering force over extended periods of time by smoothly supporting the radial load and the thrust load by preventing the entry of foreign objects, such as dust, onto the sliding surfaces without using a dust seal formed of an elastomeric resilient member.
The synthetic resin bearing according to this prior art is comprised of a synthetic resin-made upper case, a synthetic resin-made lower case, and a synthetic resin-made bearing piece having a hollow cylindrical radial bearing piece and a disk-shaped thrust bearing piece which are disposed in the upper and lower cases. The upper and lower cases are assembled by being snappingly fitted to each other. At the same time, hermetically sealing portions which exhibit labyrinth action are formed on both inner and outer peripheral sides thereof. The radial load is received between the hollow cylindrical radial bearing piece and the lower case or the upper case, while the thrust load is received between the disk-shaped thrust bearing piece and the lower case or the upper case.
This synthetic resin bearing is capable of preventing the entry of foreign objects, such as dust, onto the sliding surfaces without using a dust seal formed of the elastomeric resilient member described above, and is capable of smoothly supporting the thrust load and the radial load by a combination of synthetic resins excelling in the sliding characteristic.
However, since all the upper and lower cases and the bearing pieces which are disposed in the upper and lower cases are formed of synthetic resins, these component parts are bound to have dimensional variations due to molding shrinkage. Particularly in a case where the diameter of the hollow cylindrical radial bearing piece for bearing the radial load is large, it is necessary to provide large dimensional tolerances. Hence, it is difficult to maintain proper gaps for the snappingly fitting portions and engaging portions of the upper and lower cases in the estimation of the dimensional tolerances of the component parts. Consequently, there arise problems in that the snappingly fitting portions and engaging portions slide on each other with excessive friction, and large gaps are formed in the snappingly fitting portions, with the result that the smooth relative rotation of the upper and lower cases is hampered, and the entry of foreign objects, such as dust, into the bearing is disadvantageously facilitated. These problems become more noticeable by the temperature change at the position where the synthetic resin bearing is installed.